The present invention relates to a polygon mirror having a plurality of reflection surfaces for reflecting an optical beam, and to an optical scanning device provided with the polygon mirror. The present invention also relates to an image forming device provided with the optical scanning device.
In a conventional image forming device such as a printer, a facsimile and a copying machine, an optical scanning device is incorporated for scanningly irradiating a light beam onto a photosensitive drum for forming an electrostatic latent image thereon. The optical scanning device includes a polygon mirror having a plurality of light reflection surfaces.
Japanese Patent Application Publication Nos. Hei 7-199106 and Hei-11-295633 disclose a polygon mirror in which each light reflection surface has a writing region for reflecting light beam available for forming the electrostatic latent image and a non-writing region for reflecting light beam not available for forming the electrostatic latent image.
In the JP Publication No. Hei-7-199106, attention is drawn to a local degradation of the electrostatic latent image by a weakened power of the optical beam reflected at a specific region of each light reflection surface of the polygon mirror. That is, a fog is generated at the specific region, and the power of the light beam reflected at the specific region becomes lower than a predetermined light beam power. To avoid this problem, in the JP publication, the non-writing region is set at the specific region where the fog is generated. Accordingly, the optical beam reflected at the specific region becomes unavailable for the formation of the electrostatic latent image.
However, because the foggy region is not utilized for forming the electrostatic latent image, each reflection surface must have an increased area in order to provide an increased area of the writing region. Consequently, resultant polygon mirror becomes bulky, which in turn renders the optical scanning device and the image forming device also bulky.
In the JP publication No. Hei-11-295633, a light reflectivity of the non-writing region is set higher than that of the writing region in each reflection surface. With this structure, if a fog generation area is within the writing region, a resultant electrostatic latent image has a degraded imaging region formed by the optical beam reflected at the fog generating area. Thus, resultant visible image is also partly degraded.